In the servicing and trouble-shooting of electromagnetic equipment of various types the serviceman or technician often needs to know whether a relay or solenoid has been energized. Many such devices are enclosed or the armature is obscured by other parts so that it is impossible to determine by visual examination whether or not the relay or solenoid has operated.
It then becomes necessary to check coil voltages, continuity of contacts, etc. with a voltmeter and/or an ohm-meter. This involves the time consuming persual of wiring diagrams, the careful tracing of wiring, and the probing of energized circuits which often operate at hazardous voltages. This method is not always accurate because of the possibility of errors in the tracing of diagrams or wiring. Furthermore, if the coil of the relay is "open" the meter check yields incomplete information.
What is needed is a simple instrument which may be conveniently employed to probe the magnetic field surrounding the relay or solenoid to determine very quickly whether or not it has been energized.